As a construction machine performing an excavating and loading operation, there is a wheel loader having a bucket in a front portion of a vehicle and mainly excavating a loaded object such as crushed stones and rocks, earth and sand, or the like by the bucket so as to load on a dump truck or the like. FIG. 10 shows a side elevation view of the wheel loader.
In FIG. 10, a wheel loader 1 is provided with a working unit 5 having a boom 3 attached to a front portion of a travelable vehicle body 2 in such a maimer as to freely move in a vertical direction, and a bucket 4 pivoted to a front end portion of boom 3 in such a manner as to freely rotate in a vertical direction. The boom 3 and the bucket 4 are operated by operating levers (not shown) provided within an operating room 7 mounted on the vehicle body 2. At a time of excavating a loaded object 6 so as to load on the bucket, the boom operation and the bucket operation are alternately performed while forward moving the vehicle toward a heap of the loaded object 6. In this case, rotating the bucket 4 around a pin 8 in a clockwise direction in FIG. 10 is called a tilting operation.
A technique of semi-automatically controlling a bucket angle of the bucket 4 in correspondence to a change of a boom angle of the boom 3 so as to improve an operating efficiency, in the excavating operation mentioned above is shown in Japanese Unexamined Patent Publication No. 2000-96601. In accordance with this publication, a relation of the bucket angle with respect to the boom angle at a time of excavating is previously stored, and the bucket angle with respect to the boom angle operated by an operator is controlled so as to satisfy the relation mentioned above after a control start signal is input from the operator. That is, the bucket angle is controlled so that a detecting amount of a bucket angle detector becomes a stored target bucket angle.
However, in accordance with the prior art mentioned above, the operator determines the control start. A skilled operator can properly judge the control start timing so as to improve an efficiency of the excavating operation, however, it is hard for an operation having a low skill level to properly judge the excavation start timing. Accordingly, there is a case that the operating efficiency can not be improved because the control start timing is not properly judged, further, there is a case that the operating efficiency is reduced in some controls.
Further, in accordance with the conventional art, when the boom angle becomes a predetermined angle, the bucket is tilted by extending a bucket cylinder controlling the bucket angle for a predetermined time. However, in this control method, there is a case that the bucket angle corresponding to the boom angle can not be obtained due to a relief of a hydraulic circuit in the bucket cylinder or the like. In this case, since the bucket cylinder does not extend to a stroke end when the boom angle reaches a predetermined angle and the excavating control is finished, the tilt of the bucket is insufficient and a load scattering is generated. Further, since the bucket cylinder extends to the stroke end before the excavation control is finished, the hydraulic circuit of the bucket cylinder is wastefully relieved. Accordingly, it is impossible to control the working unit in accordance with an intention of the operator and the operating efficiency can not be improved.